The present invention relates to a gear hobbing machine in which the hob is supported without the use of an arbor.
Hobbing type gear cutters consist generally, and essentially, in a pair of coaxial heads (spindle and support) by which the hob, an axially hollow cutting tool, is supported and set in motion. In conventional machines of the type in question, the hob is carried by an arbor passing coaxially through the bore of the hob and secured by a lock nut. The arbor is provided with means by which it is fastened to the heads of the machine, and means by which to determine the angular position of the hob, i.e. its timing, in relation to at least one of the heads. In most instances such timing means take the form of keys disposed longitudinally or transversely to the axis of the hob and associated or interacting with the arbor.
The importance of the mutual angular positioning of the hob and the two heads, crucial in ensuring that teeth are cut correctly, dictates almost inevitably that each individual hob be associated permanently with a relative arbor and, more especially, aligned with the timing key in readiness to operate. Such a provision has the effect of saving a considerable amount of time when the hob is changed, since the replacement tool is already timed when fitted, as well as allowing the adoption of a semi-automatic change facility.
The means by which the arbor is secured to the relative heads may consist in threaded fasteners or quick coupling elements, or a combination of both. Clearly, in the event that the fastening means are exclusively of threaded type, for example tapped axial holes in the butt ends of the arbor engaged by bolts passing through the heads, the change operation will be effected manually; where quick release couplings are incorporated, for example by embodying the arbor with shanked ends and equipping the heads with collets, the change operation can be essentially automatic.
Whichever of the above solutions is adopted, the fundamental need to fit each hob with a respective arbor represents a notable cost, besides involving machine operatives in an additional laborious task, especially when replacing hobs of somewhat large dimensions.
Accordingly, the object of the present invention is to embody a hobbing machine in which successive hobs can be supported and rotated correctly without the constraint that each hob should be associated permanently with a relative arbor.